1. Field of the Invention
This invention relates to a method for coating inner faces of metal tubes or pipes (hereinafter referred to as "pipes") of a small diameter. More particularly, the invention relates to a method for forming a highly corrosion-resistant coating on the inner face of a long metal pipe of a small diameter, especially a diameter smaller than 2 or 3 inches, by uniformly coating the inner face of the pipe with powder of a synthetic resin with high efficiency.
2. Description of the Prior Art
Various methods for coating inner faces of metal pipes with powdery resins have heretofore been known in the art. For instance, there is known a method for coating inner faces of a pipe by inserting a long nozzle into the pipe, spraying a powdery resin from a nozzle tip while moving the nozzle from one end of the pipe to the other end, and thus forming a resin coating on the inner face of the heated pipe (see specifications of U.S. Pat. Nos. 3,016,875 and 3,245,824). The fatal defect of this method is that it is difficult to insert the nozzle into a pipe to be coated. The smaller is the inner diameter of the pipe or the longer is the pipe, the more difficult and troublesome is the operation of inserting the nozzle into the pipe. Accordingly, this method cannot be conveniently applied to a production line in which high speed operation is required.
Another known method comprises moving a vertical pipe partially heated by an induction heater or a ring-like gas burner in a fluidized bed while blowing up a powdery resin into the pipe or causing the powdery resin to flow downwardly in the pipe by the action of the fluidized bed, to thereby melt-coat the powdery resin on the inner face of the pipe and form a coating thereon (see specifications of U.S. Pat. No. 3,074,808 and Japanese Patent Publications Nos. 9947/60, 5536/61, 15080/63 and 23792/68).
There is also known a method comprising placing a preheated pipe vertically in a fluidized bed and blowing up or sucking a powdery resin into the pipe to thereby melt-coat the powdery resin on the inner face of the pipe and form a coating thereon (see specifications of U.S. Pat. Nos. 3,063,860 and 3,138,483 and Japanese Patent Publication No. 14340/68.
According to each of the foregoing methods, a powdery resin is sucked or compressed into a pipe from a fluidized bed. In this case, however, the pipe can be treated only in the vertical state because it is vertically connected to the fluidized bed. Treating or handling long pipes in the vertical state involves various difficulties when the operation is conducted on an industrial scale. Therefore, these methods are not suitable for mass production. .
As another known method, there can be mentioned a method comprising feeding as a fluidizing gas a low temperature gas maintained below 0.degree.F. into a fluidized bed, maintaining in vacuo the interior of a pipe preheated at a high temperature (350.degree. to 600.degree.F.), sucking a cooled powdery resin into the pipe from the fluidized bed and thus forming a resin coating on the inner face of the pipe (see the specification of U.S. Pat. No. 3,532,531). In this method, the entire system including the pipe to be coated must be kept in vacuo, and therefore, each joint portion must be kept air-tight. Further, since the fluidized gas is maintained below 0.degree.F., condensation of water readily occurs in the apparatus and on the powdery resin, and therefore, interception of outer air and maintenance of air-tightness are very important in this method. However, various operational difficulties are involved if the operation is conducted on an industrial scale so that the foregoing conditions are satisfied. Moreover, the equipment cost is very high.
Some methods are known in the art, according to which a coating-forming powdery resin is passed through the interior of a metal pipe with a gas stream (air stream) and coating of the inner face of the metal pipe is accomplished by the heat retained by the preheated pipe or by using an induction heater.
For instance, the specification of Japanese Patent Publication No. 25152/65 discloses an apparatus in which a powdery thermoplastic resin is compressed or sucked together with air into a preheated pipe being rotated in an inclined manner to cause the resin powder to stick on the inner face of the pipe and excessive powder remaining in the pipe is blown away by blowing-in of compressed air. In the invention disclosed in this prior art reference, in order to obtain a uniform coating on the inner face of a pipe it is indispensable to rotate the pipe during feeding of the powdery resin. The reason is that since the concentration of the powder fed is low, not exceeding several grams per liter, the majority of the power flows along the lower face portion during passage through the interior of the pipe and if the pipe is not rotated, the thickness of the resin powder coated on the inner wall of the pipe differs considerably between the upper face portion and lower face portion.
A similar technique is disclosed in Japanese Patent Application Disclosure No. 17856/73. According to this method, a powdery resin in great excess of the amount necessary for coating is fed into a preheated metal pipe together with a gas and then only the gas is fed into the metal pipe, whereby a uniform resin coating is formed on the inner face of the metal pipe. More specifically, a powder in an amount 3 to 20 times as large as the amount to be applied as coating is fed together with a gas within a certain coating period and passed through the metal pipe. At this time, the powder concentration is lower than several grams per liter (more concretely, below 7.1 g/l). Also in this method rotation of the pipe is indispensable, and since the powder concentration is as low as mentioned above and the powder feed rate is relatively high, namely 12 to 30 m/sec., in order to coat the resin powder on the inner wall of the pipe with high efficiency, it is necessary to preheat a pipe to be coated to a temperature much higher than the melting point of the resin, and provision of an apparatus of large dimensions is necessary for attaining a high gas speed for carrying the resin powder. Still further, it takes as long as shout 20 seconds to complete coating of a pipe of 5.5 m standard length.
A similar method is known from Japanese Patent Application Disclosure No. 8789/72. In this method, a gas stream containing a resin powder and a gas free of a resin powder are circulated independently, and at the time of coating, the resin powder-containing gas stream is made to flow through the interior of a pipe to be coated, and heating and cooling of the pipe are controlled by an induction heater and a subsequent water cooler so that the resin powder melt-coats on the inner face of the pipe. Coincidentally with termination of coating of the inner face of the pipe, the powder-free gas stream is made to flow through the pipe by the switch-over of a change-over valve to blow away the remaining excessive powder from the interior of the pipe, and coating of the inner face of the pipe is thus completed. Also in this method, the powder concentration in the gas stream passing through the interior of the pipe at the time of coating is as low as several grams per liter, and therefore, it is indispensable to rotate the pipe to be coated or to provide a high speed gas stream, in order to obtain a uniform coating on the inner face of a pipe. This method is characterized by provision of an induction heater and a pipe-cooling device interlocked with the induction heater. In the induction heating system, however, the heating rate is limited mainly for economical reasons, and this method is defective in that in order to obtain a sufficient moving speed, for example, several tens of meters per minute, it is necessary to provide an expensive apparatus of large dimensions. Moreover, also in this method it takes as long as 30 to 60 seconds to complete coating of a pipe of 5.5 m standard length. Accordingly, it is apparent that the inner face coating method using an induction heating system is not suitable for coating long pipes.
As is apparent from the foregoing description, the each known methods has its defects and shortcomings, and none of them is suitable as an industrial system for mass production.